Fuel efficiency and component package envelopes are becoming a driving force in the design of vehicle drivelines. Specifically, designers are challenged by the need to provide the all-wheel drive capabilities of larger sport utility vehicles (SUVs) in smaller more compact vehicles to achieve better fuel efficiency while maintaining traction capabilities. Motor vehicles may be driven by a transmission that transmits rotational torque from a power head, such as an engine, to a power transfer unit (also known as a power take-off unit) through a torque-transmitting shaft. The power transfer unit ultimately drives a plurality of axles that can be divided into those with a hang-on four-wheel drive, wherein a primary axle is driven permanently and a secondary axle is connected, if required, and those with a permanent four-wheel drive or all-wheel drive, wherein all axles are driven permanently as drive torque is split between all wheels. The design of the driveline is largely influenced by the arrangement of the engine in the motor vehicle, i.e. whether it is arranged in the front or at the rear and whether it is positioned in the longitudinal or transverse direction. At the same time, stringent packaging requirements exist regarding size, weight, and assembly costs of such systems.
Power transfer units are commonly utilized in front-wheel drive based all-wheel drive systems. A power transfer unit transmits the torque from the transmission to a propshaft, which in turn delivers power to the rear wheels. Most power transfer units are always in a ready state, commonly controlled by a slipping clutch near the rear axle, and yet are utilized only a small fraction of the time during driving. However, in this “ready state”, the existing power transfer units exhibit a full time drain to fuel efficiency with only a part-time benefit to traction.
In typical four-wheel or all-wheel drive based layouts, the power transfer unit is always rotating when the vehicle is in motion, creating energy losses due to gear mesh, rotating inertias, bearing drag, as well as oil churning. These losses reduce the fuel economy and may create premature wear on the rotating assembly. Additionally, typical power transfer units are bulky and include rotating components that were originally configured for larger vehicles, which prohibit interchangeability in smaller motor vehicles. Thus, there exists a need for a power transfer unit that minimizes the energy losses to increase fuel efficiency while maintaining a small package envelope for use in various motor vehicle platforms.